Technical Field
The present invention relates to a freezing detection device to be applied to e.g. a beverage cooling apparatus for making beverages (e.g. beer) cold.
Description of the Related Art
Conventionally, at a restaurant or the like, when a beer server is used for pouring beer into mugs or glasses to serve customers, an ice thermal-storage cooling apparatus has been typically used for cooling the beer. For example, the ice thermal-storage cooling apparatus cools water (beverage cooling liquid) stored in a water tank provided inside the cooling apparatus by an evaporator of a vapor-compression refrigerator located in the water tank so that ice is formed around the evaporator for thermal storage, and cools beer by allowing the beer guided from a beer keg to pass through a pipe placed inside the water tank so that the beer is cooled to an appropriate temperature for drinking (e.g. about 5° C.). The use of ice thermal storage enables required beverage-cooling capability to be ensured while enabling a capacity of the vapor-compression refrigerator to be smaller.
In order to exert control in such a manner that the amount of ice formed around the evaporator reaches a predetermined amount, such an ice thermal-storage cooling apparatus is provided with a freezing detection device for detecting that the amount of ice formed around the evaporator has reached the predetermined amount. A conventional freezing detection device is provided with a pair of electrodes at a position corresponding to a desired amount of ice, and determines whether or not the water has frozen around the electrodes on the basis of a difference in electrical conductivity between water in a liquid state and water having frozen into a solid state, thereby determining whether or not the desired amount of ice has been formed.
Such a conventional freezing detection device, however, sometimes makes an incorrect determination that the desired amount of ice has been formed in spite of the fact that the desired amount of ice has not been formed, due to decrease in electrical conductivity of the water stored in the water tank, when the water stored in the water tank has changed in quality with the passage of time, and the amount of electrolyte contained in the water has decreased.
Furthermore, there are some sorts of beverage having an appropriate temperature for drinking (e.g. −2° C.) that is lower than the temperature to which a conventional ice thermal-storage cooling apparatus can cool the beverage. As an apparatus for cooling such beverages to the appropriate temperature for drinking, there is known an apparatus which utilizes an antifreeze liquid whose freezing temperature is lower than that of water, instead of water, as a beverage cooling liquid and cools the antifreeze liquid by a vapor-compression refrigerator to achieve the temperature of beverage which is lower than that achieved by a conventional ice thermal-storage cooling apparatus.
When using an antifreeze liquid as a beverage cooling liquid in such a manner, it is difficult to determine on the basis of a difference in electrical conductivity whether or not the antifreeze liquid has frozen because the antifreeze liquid, which is usually higher in electrical conductivity than water, exhibits a high electrical conductivity even in a frozen state.
Japanese Patent Application Laid-Open No. 2011-73775 describes a beer server including a freezing sensor located in the vicinity of an evaporator pipe in a water tank and configured to detect on the basis of change in electrical resistance (electrical conductivity) of water whether or not a predetermined thickness of ice has been formed around the evaporator pipe; and a beer server including a temperature sensor for freezing detection configured to detect whether or not a predetermined thickness of ice has been formed around an evaporator pipe through the use of a difference in temperature between cooling water and ice.
Patent Document 1: Japanese Patent Application Laid-Open No. 2011-73775 (¶¶ 0047, 0048, FIG. 2, ¶ 0080, FIG. 12)
An object of the present invention is to provide a freezing detection device capable of determining whether a liquid whose freezing is to be detected has frozen or not, irrespective of electrical conductivity of the liquid whose freezing is to be detected.